1. Field of the Invention
The present invention relates to a fuel cell system and in particular, to the control of a fuel cell system in the event of a power failure.
2. Discussion of the Related Art
As one form of fuel cell systems, there has been known one which is described in Japanese examined patent No. 08-21408. As shown in FIG. 1 of the Japanese patent, the fuel cell system is provided with two resistances 10, 15 having low and high resistance values which are connected to direct current output terminals of a fuel cell 1 in parallel relation for discharging residual voltage at the time of the operation stop of the fuel cell, a normal closed switch 14 connected to the resistance 15 with the high resistance value for making the resistance 15 active or passive, and a normal open switch 9 connected to the resistance 10 with the low resistance valve for making the resistance 10 active or passive.
With the system, in the event of a power failure of the control power supply, both of the high resistance 15 and the low resistance 10 are made to be active by the manipulations of the switches 14, 9 immediately after the power failure to lower the residual voltage quickly to an electric potential for the protection of the power supply. Then, the low resistance 10 is made to be passive to lower the residual voltage slowly.
In the fuel cell system described in the aforementioned Japanese patent, since the residual voltage is lowered (i.e., electric power is used) regardless of the output voltage of the fuel cell 1 (or the service voltage for loads in the power system) in the event of a power failure, there is a risk that the residual voltage cannot be lowered appropriately. Further, there arises a problem that the use of electric power by the resistances 10, 15 for a long time causes the resistances 10, 15 to become high temperatures.